1. Field of the Invention
The present invention relates to renewable energy production, as well as devices, methods, compositions, systems and other means for manufacturing and using an integrated architecture for charge production from radiation and thermal energy.
2. Description of Related Art
Heat is considered to be a form of waste in a vast array of devices and processes in myriad industries and applications (e.g., transportation, heat and power production, manufacturing and electronics). Microprocessors, for example, generate heat as they consume electrical energy to perform their intended functions. Such devices are designed at great effort and expense to dissipate heat because too much of it diminishes the function of the microprocessor and its related components. In another example, a passenger car uses only about a third of the stored chemical energy in gasoline for power. The balance of the energy released during combustion must be dissipated as waste heat to avoid catastrophic component failure.
There are many known methods and devices that recover heat by conductive, convective and/or radiative means. Nevertheless, about 60% of all of the energy consumed in the U.S. is dissipated to the environment in the form of waste heat. 90% of that energy is released at low grade temperatures (less than 470 K) that are insufficient for most known heat recovery technologies to operate cost-effectively. Such low temperature resources are generally insufficient to justify the capital and other requirements of conventional recovery methods; dissipation into the environment has simply been more cost-effective.
Heat presents further challenges in photovoltaic (PV) and thermophotovoltaic (TPV) devices which convert radiation to electricity, and in photocatalytic (PC), photoelectrochemical (PEC), and thermophotocatalytic (TPC) devices which convert radiation to charge carriers that facilitate chemical reactions. All such known devices and methods are most notably limited by material composition, geometry and other factors which frustrate charge separation and transport and permit significant electron-hole pair recombination to occur. Unless rapidly separated and transported from the cell, photogenerated charges recombine and contribute to thermalization and, in the case of low band gap semiconductors, photocorrosion. Increasing the thermal energy of the system further reduces overall photoconversion efficiency (PCE) by lowering charge mobility and promoting additional recombination; and, photocorrosion has historically impeded development of infrared photovoltaics since the low band gap semiconductors required for such applications are susceptible to oxidation and/or reduction by electron-hole pairs following recombination for the want of rapid separation and transport.
Attempts have been made to decrease these losses, such as by use of dopants, dyes, quantum dots and other substances, structures and device designs that shift the bandgap energy of solar cells; and, in the case of TPV devices, through the use of separate structures to absorb, reemit, reflect and/or filter radiation and thermal energy. However, such attempts have not to date achieved commercial success. For example, high material purity is necessary to achieve sufficient mobilities to avoid recombination while crossing the considerable distances (e.g., upwards of several hundred microns) required for effective absorption in crystalline silicon devices. Fabrication of cells of such purity requires significant cost. In another example, while it is known that quantum dots can be used to absorb infrared photons, the resulting photogenerated charges have very short lifetimes (e.g., femtoseconds)—timescales far too short for such methods to be effective using devices incorporating known charge separation and transport techniques.
Moreover, thermalization is inevitable in such applications notwithstanding the relative speed and degree of separation and transport. In response to AM 1.5G solar radiation, crystalline silicon cells with a 1.1 eV absorption edge are able to absorb photons representing approximately 77% of the solar spectrum (with wavelengths ranging from about 350 nm to about 1.1 μm); the remainder (with wavelengths ranging from about 1.1 μm to 1 mm) heats the cell and its surrounding structures and environment. Further, almost all of the photons in the flux absorbed by the cell have energies greater than the bandgap energy of silicon, and produce both an electron-hole pair and thermalization losses upon absorption. Multiple exciton generation (MEG) methods have been demonstrated in laboratory settings which offer the potential of preventing at least some thermalization by converting surplus (above band gap) energy into additional charge. However, while single excitons produced upon absorption of visible radiation have lifetimes on the order of hundreds of nanoseconds to microseconds, multi-excitons decay much more rapidly and typically recombine within picoseconds—much too fast for MEG to be effective using devices incorporating known methods for charge separation and transport.
In total, more than about two thirds of the solar flux striking a conventional PV device is converted to thermal energy. That energy will eventually dissipate as the system and its surroundings achieve thermal equilibrium. However, the rate and degree to which the resulting dissipation occurs is in part a function of the ambient temperature in the local environment, which is itself directly and indirectly heated by the vast majority of the incoming solar flux that never comes in contact with the surface of the cell. Most of that energy is absorbed and stored by the environment (e.g., air, water, land, buildings, PV housings, and so on) in the form of thermal energy which is then released at low grade temperatures by convection, conduction and, importantly, emission of infrared radiation—in every direction, day and night, 24 hours a day, and 365 days per year. Humanity then adds to those emissions by engaging in activities that create waste heat, which is likewise discharged by convection, conduction and radiation. Conventional attempts to access and convert such emissions into electricity have been limited and unsuccessful.
Materials with temperatures above absolute zero emit radiation with a characteristic frequency distribution that is dependent upon the temperature of the emitter. The flux density, or radiant intensity (energy radiated per unit time, or power, per unit of area, W/m2), I(T), of the emitter is given by the product of the emissivity (ε) of the emitter (a real number between 0 and 1), the Stefan-Boltzman constant (σ=5.67×10−8 W m−2 K−4), and the fourth power of the temperature (T) of the emitter in degrees Kelvin (I(T)=εσT4). The thermal emission spectrum of the emitter is then described using Planck's law of black body radiation, which is a function of the speed of light (c), Planck's constant (h=6.626×10-34 Js, or 4.14×10-15 eVs), Boltzmann's constant (k=1.381×10-23 J K−1), wavelength (λ), and temperature (T) in degrees Kelvin (R(λ)=(2πc2h/λ5)(1/ehc/λkT−1)). Integration of the Planck equation over the wavelength range yields the radiant intensity I(T). The wavelength at which the maximum irradiance occurs is inversely proportional to the temperature, and is given by the Wien displacement law (λmaximum (μm)=2,897.8/T).
The radiant intensity of a material at room temperature (300 K) is approximately 500 W/m2 or 0.05 W/cm2 and the maximum emission occurs at approximately 10 μm. These emissions fall exclusively within the infrared spectrum: none have wavelengths ranging from 0.75 to 1.4 μm (near-infrared, or NIR) or from 1.4 to 3.0 μm (short-wavelength infrared, or SWIR); 22.5% ranges from 3.0 to 9.0 μm (mid-wavelength infrared, or MWIR); 38.9% is between 9.0 and 15.0 μm (long-wavelength infrared, or LWIR); 21.3% is between 15 μm and 20 μm (far infrared, or FIR); and the remaining 17% is between 20 μm and 1 mm (also FIR). No known commercial system exists for the conversion of such low grade emissions.
Current thermophotovoltaic (TPV) systems are fundamentally limited by inadequate separation and transport capabilities. Such systems attempt to avoid recombination and photocorrosion by harvesting a relatively narrow range of IR radiation (typically NIR) emissions from an intense (high temperature) proximate source of radiation. It is common for such systems to first use the source to heat an emitter (e.g., a tungsten plate) with emissivity characteristics which in turn limits the incoming flux to wavelengths that the cell can more readily convert (e.g., NIR radiation using a GaSb cell with a 0.75 eV band gap). The spectral radiancy can be further restricted by placing a pass band filter between the emitter and the cell to reflect all but the desired wavelengths away from the cell. Despite these protections, the radiation incident upon the cell that is not converted to electricity heats the device, which reduces the charge carrier mobility and PCE of the semiconductor (currently available TPV systems do not directly convert thermal energy into electricity, such as in the case of thermoelectric devices). As a result, known TPV devices are limited to NIR systems designed to operate from about 1,300 to 2,000 K.
Fabrication methods for vertically oriented anatase nanowire arrays on transparent conductive substances have been described in US Patent Publication No. 2013/0048947 (947), which is in its entirety incorporated by reference herein, teaching synthesis of an architecture for use in PV, PC and other applications: vertically-oriented, one-dimensional (1-D), mono- or polycrystalline, anatase TiO2 nanowires in communication with a common transparent substrate or transparent conducting oxide (TCO) coated substrate; and which are intercalated with p-type semiconductors (e.g., quantum dots) such that separation occurs orthogonally over the nanometer-scale distance at each p-n junction following absorption along the micron-scale length of the nanowires.
The '947 application describes the means to simultaneously orthogonalize light absorption and charge separation while rapidly transporting photogenerated charge. Since there is no perfect light absorber, absorption is more fully achieved with thicker layers of light absorbing material. However, thicker layers generally result in slower or less efficient charge separation that in turn leads to unwanted charge recombination. This tradeoff between light absorption and efficient charge transfer has, for example, limited the nanoparticle-based photoanode thickness of modern liquid-based dye sensitized Gråtzel solar cells (DSSC) designs to approximately ten microns; although a thicker photoanode layer enables greater light absorption, any benefit obtained with increased light absorption is offset by increased recombination of the photogenerated charges. In contrast, by orthogonalizing absorption and charge separation with structures that also facilitate rapid charge transport (e.g., to underlying electrical contacts), separation occurs over comparatively much smaller distances (i.e., on the order of nanometers versus microns) and absorption can be increased without incurring the sort of recombination penalties that plague known devices. It is desirable to provide for additional or alternative synthesis techniques, as well as further aspect compositions, structures, shapes and orientations resulting in integrated architectures which facilitate conversion of radiation and thermal energy into electrical and other forms of energy.
A need exists for compositions, devices, methods and systems which separate and transport photogenerated charges at rates which minimize or prevent recombination, thermalization and other losses while enabling multiple exciton, broad spectrum and thermal energy conversion.